How to design a file container in C++?

Question

My plan is to build a fileContainer that consists mainly of a std::map that associate specific files to an ID. Each file has for attributes a ofstream, a path (string) and a few other information.

The problem is that an ofstream cannot be copied (Why copying stringstream is not allowed?) and cannot even be moved (Why can't std::ostream be moved?). One can therefore not create file objects to then insert them into the map of the fileContainer.

What I am trying to do is something like

file f(arguments...); // create a `file`
FC.insert(f);         // Note the FC is a global singleton of class `fileContainer`
...
{
   file& f = FC.getFile(fileID); // find a specific file in the file container
   f.open();
   f.write(s1);
   f.write(s2);
   f.write(s3);
   f.close();
}

I fail to see how such functionality could be achieved without having to copy or move a stream. Can you help me out to build this type of functionality?

Response to @πάνταῥεῖ comment

My code can produce about 20 different types of files. The types of files (which I above call ID) that are actually being produce depends upon the user input.

My goal is to have a fileContainer in which, I insert file objects. Those file objects are created while I read the input. Each file object match to a file ID. For any given file object a single file is being produced but for others, several files are being produced (information that can be gathered only during the process).

During the process, I would just look at whether a given ID is present in the fileContainer and if it is, then I write to it the associated file.

I already have a working version of the code, the issue is that it does not compile on every machine (looks like some compilers are fine with moving streams while others aren't)

What's the use case you want to tackle? Sounds like a XY problem for me. — πάντα ῥεῖ, May 31 '17 at 19:05
Right... I understand your comment. Thanks. I tried to respond to it directly in the post (see Response to @πάνταῥεῖ comment) — Remi.b, May 31 '17 at 19:17
Your link says that `ofstream` IS moveable, only `ostream` is not. — Ben Voigt, May 31 '17 at 19:24

Lightness Races in Orbit · Accepted Answer · 2017-05-31T19:25:31.030

2

If you only need `std::ofstream`

No problem! These are moveable.

If you need any `std::ostream`

Since std::ostreams cannot be copied or moved, your only option is to go back in time and store pointers instead. Use dynamic allocation to get fine control over the lifetime of your stubborn stream objects.

Abstracting away your custom container, a basic example looks like this:

std::vector<std::unique_ptr<std::ostream>> container;
container.push_back(std::make_unique<std::ostringstream>());
container.push_back(std::make_unique<std::ofstream>("/tmp/lol"));

…and so forth.

Nice, huh? #c++

edited May 31 '17 at 19:25

answered May 31 '17 at 19:20

Lightness Races in Orbit

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This is what I would suggest, however there is another way peculiar to iostreams. – Ben Voigt May 31 '17 at 19:23
2

@BenVoigt: By all means reveal the secret! – Lightness Races in Orbit May 31 '17 at 19:23
@BenVoigt: Is it something to do with stealing buffers (via `rdbuf`) in a hypothetical `file` move constructor? And is it impervious to [the problems Howard became concerned about back when streams _were_ going to be moveable](https://stackoverflow.com/a/20774666/560648)? – Lightness Races in Orbit May 31 '17 at 19:23
Yeah, `rdbuf()` is exactly it. You then have a slight problem with lifetime management of the buffer objects. Anyway, the linked answer by Howard Hinnant says that ofstream is moveable... off to check that. – Ben Voigt May 31 '17 at 19:25
@BenVoigt: Yeah I think the problems with that are why `std::ostream` _isn't_ moveable. – Lightness Races in Orbit May 31 '17 at 19:25
Another option is to store `filebuf` objects in the map (they're moveable) and let the streams be transient (temporarily associated to the `filebuf`, then detached and the stream gets destroyed while the `filebuf` remains) – Ben Voigt May 31 '17 at 19:28
@BenVoigt: That's not terrible – Lightness Races in Orbit May 31 '17 at 19:29

How to design a file container in C++?

1 Answers1

If you only need std::ofstream

If you need any std::ostream

If you only need `std::ofstream`

If you need any `std::ostream`